You are using mixed metaphors when you say fuse/circuit breaker. They are entirely different in many ways. Here's some clarifications that will certainly only confuse the issue:
A fuse is a device that destructs when overloaded. Once it does it's job, it has to be replaced. In RV's these come in several shapes, tubular glass and two leg plug in are the most common. You can usually find out what they control by removing one and looking for what isn't working.
2. Circuit Breakers
come in several different flavors:
Switch type Circuit Breakers:
There are usually on your AC panel and protect the entire AC input as well as separate loads such as ac outlets, the converter and the air conditioner. They look a little like a wall switch and, when overloaded, they trip OFF to a mid point position and have to be turned fully OFF than back on to reset. Although commonly used as ON/OFF switches, using them as such will reduce their trip point and cause premature tripping.
Automatic Resettable Circuit Breakers
These usually look like a small cube shaped can with two threaded terminals. They are designed to trip OFF when rated load is exceeded and, after a short time, automatically reset. Some will have a small button to push to reset, but most in RV's do this automatically. The ones shown in the link all seem to operate automatically and, except for trip rating, are electrically identical.This tripping can usually be heard as a metallic clicking sound. Again, repeated tripping will reduce the trip point. About the only way their purpose can be identified is by disconnecting one of the wires and finding out what stops working. Unless one is travelling to a planet beyond Jupiter, reliability is usually considered high enough that carrying a spare isn't usually necessary.
About your problems: You do have a 20+ year old trailer and a lot of different electrical
problems can be traced to loose connections, often overloaded circuit breakers and, the biggie, bad grounds. It's sometimes takes a lot of time and patience to locate the culprits, but looking at each and every connection and ground you can find it a good start.
As far as Scotch-Loc connections: I usually replace every one I come across in older trailers. As I am sure you will hear, some like em', I don't.
BTW: Replacing that original Magnetek converter with a modern power center/converter/smart charger will head off future failures that just cant be fixed in the field. It's old school, it's old and it's tired. Look at the Progressive Dynamics PD-4045 power chassis. Less than $200 and fairly easy to install.